Twelve studies are designed to continue our investigations of automatic and cyclic hindlimb movements controlled by lumbosacral centers. Using normal and cord-transected cats, we will study the intralimb coordination of three automatic limb behaviors: locomotion, paw-shake responses, and scratching. Five hypotheses provide five aims. For Aims 1 and 2, we will examine relationships between neuromuscular patterns and limb dynamics during several forms of locomotion. Hypotheses are: 1) Some details of intralimb coordination are dependent on motion-dependent feedback. We propose that activity patterns of bifunctional muscles which decelerate the hip and knee during swing are related to moments due to leg angular acceleration which increase with locomotion speed. Thus, the activity of these muscles will be enhanced during fast locomotion and absent or reduced during slow walking (cat), airstepping (spinal cat), and spontaneous fictive locomotion (decerebrated-spinal cat). 2) At the spinal level, circuits exist for alternative forms of walking. Backward walking requires a dissociation of hip from knee and ankle motions, using mixed synergies in which hip flexors are coactive with knee and ankle extensors. We will assess backward walking in normal and spinal cat. For Aim 3, we will examine relationships between neuromuscular patterns and limb dynamics during the paw-shake response; the hypothesis is: the mixed synergy, typical of the paw shake, relys on motion-dependent feedback. During paw shaking, knee extensors are coactive with ankle flexors, and this novel, mixed synergy is sensitive to motion-dependent feedback related to moments caused by paw angular acceleration. Three studies will examine the response during trials in which motion-dependent feedback is distorted (limb casted, paw weighted) or eliminated (fictive). The fourth study focuses on how segmental trajectories "self-organized" so that stable, limit-cycle oscillations emerge. For Aim 4, we will develop a model to study scratching in spinal cat. The hypothesis to be tested is: spinal cats will develop hindlimb scratching with muscle synergies similar to those of normal cats. For Aim 5, we will examine effects of selected putative transmitters on the control of hindlimb motions. The hypothesis is: putative transmitters have specific effects on various patterns of automatic limb actions. We propose that a single array of unit-burst generators may provide coordinative circuits for automatic motions, and if the same units are used, mechanisms for setting appropriate connections among them may exist. Since networks may rely on different transmitters, we will study the effects of intrathecal injections of various putative transmitters and their agonists or antagonists on limb behaviors in spinal cats. The spinal cat offers a unique animal preparation in which to study the capacity of local lumbosacral circuits to control a variety of hindlimb movements. Knowledge about the interaction of intersegmental dynamics and the neuromuscular pattern is fundamental to our understanding of the role of spinal mechanisms in regulating purposeful and adaptive movements of the limbs. Our unique approach of combining the study of limb dynamics and neuromuscular patterns has provided important insights about the requirements for neural control at the spinal level. This fundamental knowledge will be useful in assessing the capacity of lumbosacral cord to regulate functions after spinal trauma or lesions.